1. Field of the Invention
The present invention relates to an antenna mainly used in a wireless device for mobile communication and the like.
2. Background Art
Recently, demand for wireless devices for mobile communication such as a portable telephone has increased rapidly. An antenna mounted on a wireless device is one of the important devices affecting the performance of the wireless device. Antennas have been required to be small and thin in size and have a reduced effect on a human.
Herein, such a conventional antenna mounted on a portable telephone as a wireless device is described with reference to a drawing.
FIG. 5 is a schematic view showing a conventional antenna mounted on a portable telephone. In FIG. 5, conventional antenna 6 includes rectangular ground plane 3, feed point 4 disposed in the vicinity of a corner of ground plane 3, antenna element 1 coupled to feed point 4, variable reactance element 5 disposed on ground plane 3, and parasitic element 2 coupled to variable reactance element 5.
The above-mentioned conventional antenna 6 switches values of variable reactance element 5 by using a switching processing circuit and the like, between a value of variable reactance element 5 showing omnidirectional radiation characteristics during a standby time and a value of variable reactance element 5 showing directional radiation characteristics by coupling antenna element 1 and parasitic element 2 to each other so that the radiation of electric waves to the side of a human is reduced during a talk time.
As mentioned above, the conventional antenna 6 changes the radiation characteristics of antenna 6 between during a standby time and during a talk time by changing the value of variable reactance element 5. Thus, an excellent radiation characteristic is kept in each state. An example of prior art information related to the invention of this application includes Japanese Patent Application Unexamined Publication No. 2005-295002.
In the conventional antenna 6, by changing the value of variable reactance element 5 during a talk time, antenna element 1 and parasitic element 2 are coupled to each other so as to strengthen the directivity of antenna 6, thereby reducing the radiation of electric waves to the human side that is brought to the ear during a talk time. As a result, it is possible to reduce SAR (Specific Absorption Rate) that is an index showing the degree of the effect of electromagnetic wave on the human.
However, in order to reduce SAR, it is necessary to change values of variable reactance element 5 between a standby time and a talk time. Therefore, it is necessary to add a switching processing circuit and the like.